A semiconductor-employed switching device (transistor, thyristor, etc.) or rectifier device (diode) is widely used as a power inverter or converter circuit device. Under the present circumstances, a more compact device with lower losses is preferable for such semiconductor device for power applications in order to meet future demands for higher power. While silicon has conventionally been used widely as a semiconductor material, wide band gap semiconductor materials having higher breakdown fields are being developed as next-generation semiconductor materials in light of the present circumstances. Since what is called wide band gap semiconductor materials such as diamond, SiC, group-III nitride semiconductor, etc. are expected to have low on-state resistance and high breakdown voltage for their material properties, significant size reduction and reduction in losses of a power controller are expected by constituting a semiconductor device for power applications using these materials.
Requirements on properties for such power diode include: (1) small leakage current during reverse blocking; (2) high breakdown voltage during reverse blocking; (3) large output current at forward conduction; (4) short reverse recovery time at shutoff; (5) high peak surge current value; and the like. Of course, a diode made of a wide band gap semiconductor material is required to meet these requirements.
Conventionally, what is called a vertical diode is generally used which conducts in a direction passing through a semiconductor substrate so as to ensure the requirements (2) and (3).
Conventionally practical, silicon-employed P—N junction diode and its modified P-i-N junction diode have advantages of high breakdown voltage at application of a reverse-bias voltage and high output current density at forward conduction because of the occurrence of carrier injection from both P and N sides, but also have a drawback of long reverse recovery time at shutoff, that is, the above requirement (4) is not satisfied.
On the other hand, a silicon-employed Schottky barrier vertical diode is also in practical use. Such diode has an advantage in that a reverse current at shutoff does not occur in principle, but has drawbacks of having large leakage current and low breakdown voltage at application of a reverse-bias voltage and low peak surge current. That is, the above requirements (1), (2) and (5) are not satisfied.
To improve the drawbacks of such silicon-employed diodes, a SiC-employed Schottky barrier vertical diode has been developed and is publicly known (cf. “P-Type 4H and 6H—SiC High Voltage Schottky Barrier Diodes” R. Raghunathan and B. J. Baliga, IEEE ELECTRON DEVICE LETTERS, Vol. 19, pp. 71-73 (1998)) (hereinafter called “Raghunathan's article”).
Further, a high electron mobility transistor (HEMT) made of a group-III nitride material having what is called a field plate structure for achieving high breakdown voltage is publicly known (cf. “Design and Demonstration of High Breakdown Voltage GaN H Electron Mobility Transistor (HEMT) Using Field Plate Structure for Power Electronics Applications” W. Saito et al., Japanese Journal of Applied Physics Vol. 43, pp. 2239-2242 (2004)) (hereinafter called “Saito's article”).
A SiC-employed Schottky barrier vertical diode as disclosed in the Raghunathan's article achieves the effect of increasing the breakdown voltage unlike a silicon-employed one, however, the drawbacks of not meeting the requirements (1) and (5) have not been solved so far.
SiC single crystal includes many crystal defects (specifically, tubular voids, what is called micropipes) and thus disadvantageously makes it difficult to manufacture with stability a device of relatively large area that can ensure sufficient output current, resulting in poor yields in manufacturing process.
Further, since a SiC-employed P—N junction diode causes carrier recombination resulting from such crystal, the output current is more likely to be limited, so that the above requirement (3) is not satisfied.
In terms of handling, it is held difficult to use a substrate having a thickness smaller than about 100 μm in manufacturing steps including semiconductor processing and assembly. In the case of a vertical diode, the thickness of substrate is directly reflected in the gap between electrodes. Since it is difficult to reduce the gap between electrodes in terms of handling, a problem arises in that a series resistance resulting from semiconductor layers cannot be sufficiently reduced.
A vertical diode made of a group-III nitride semiconductor instead of silicon or SiC is expected in principle to present properties equal to or more excellent than the SiC-employed one. When forming a vertical diode, a single crystal substrate having conductivity needs to be used as a substrate, however, a single crystal substrate of group-III nitride semiconductor is very expensive. In addition, when a device is configured as a P—N junction diode, P- and N-type conductive regions need to be formed inside a semiconductor layer. In either the P—N junction type or Schottky junction type, what is called a field limiting ring (FLR) needs to be provided. Accordingly, in either type, P- and N-type conductive regions need to be formed essentially. The use of group-III nitride semiconductor, however, arises a problem in that it is not easy to obtain a P-type conductive layer presenting high hole density that is applicable to a power diode.
Further, it is difficult to derive a configuration that achieves improved properties in a horizontal diode such as compatibility between high breakdown voltage and short reverse recovery time, from the HEMT having a field plate as disclosed in the Saito's article.